Recently, i received several emails and comments from readers (students) to share more challenging Mole Calculations (or Mole Concepts) questions in this chemistry blog. So here we go………..
Question:
What would be the concentration of the hydrochloric acid produced if all the hydrogen chloride gas from the reaction between 50 g of pure sulfuric acid and an excess of sodium chloride was collected in water, and the solution made up to a volume of 400 cm3 of water?
Based on equation: NaCl(s) + H2SO4(aq) –> NaHSO4(s) + HCl(g)
Now, try it out and leave your suggested answer (and possibly your solution/steps) in the comment section below.
The best way to learn is NOT by reading or looking, instead it is by Taking Actions.
I look forward to seeing you answers. Cheers! Enjoy! =)
In the previous blogpost, we have discussed on the Common Errors on Calculating Empirical Formula, when given Composition by Mass. Today we shall go one step further and see how we can determine the Molecular Formula from an Empirical Formula.
The Molecular Formula shows all the atoms of each element contained in one molecule.
Empirical Formula shows the simplest ratio of the different types of elements present in a molecule.
For example, Hexane has Molecular Formula of C6H12 and Empirical Formula of CH2.
As such, Molecular Formula is a simple multiple of the Empirical Formula:
Molecular Formula = n X Empirical Formula
where n = 1, 2, 3, etc
The molecular formula can be calculated from:
- Empirical Formula
- Relative Molecular Mass, Mr of Compound
Let’s take a look at an example.
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One of the most common errors in have observed for GCE O-Level Chemistry students is when they are Calculating Empirical Formulae from Composition by Mass.
Take the following question that was given to my students:
Question:
Calculate the empirical formula of a compound that has the composition: 48.8% carbon, 13.5% hydrogen and 37.7% nitrogen.
After calculations, one of my students came up confidently with the answer of C2H5N.
- which is the Incorrect Answer.
After asking her to present her working to the class, i we realised that she made a mistake when trying to round off final numbers.
Let’s take a look at my suggested answer & then see how she made the mistake – which is a Common Error for many Chemistry students year-after-year.
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In the previous blogpost, we have discussed on the concepts surrounding Limiting & Excess Reactants, as well as look into one example.
Now, it is time for you to try out a question on your own.
Quick Check 1:
In the presence of a catalyst and at suitable temperature, 200cm3 of sulfur dioxide was reacted with 200cm3 of oxygen to form a gaseous product, sulfur trioxide. What is the total volume of gases remaining in the reaction vessel after the reaction? (2 marks)
Clue: You need to write a balanced chemical equation first!
PS: Do write down your answer in the comment section below. Showing of working is even better for your learning purpose.
Concepts of Limiting & Excess Reactants are so commonly tested in school exams as well as ‘O’ Levels Chemistry examinations. However, many students seemed to have very weak foundation of the essential concepts surrounding it.
Before we even visit any question, it is important to understand the definition of both reactants.
Limiting Reactant:
- The reactant that is completely used up in a chemical reaction
- It determines or limits the amount of products formed in a reaction
Excess Reactant:
- The reactant that are not used up in a chemical reaction
- There is always an excess at the end of the reaction, stays unreacted
Example:
Reaction of hydrogen gas with chlorine according to the equation below:
H2 (g) + Cl2 (g) –> 2HCl (g)
We can see that 1 mole of H2 reacts with 1 mole of Cl2 to form 2 moles of HCl
Scenario A: When 1 mole of H2 added to 1 mole of Cl2
Results: 2 moles of HCl are
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With 1-week March School Holidays in Singapore coming to an end, we have also successfully conducted our 3 Days annual Mole Calculations Mastery Workshop 2010 during this holiday period.
The 3 days was fun-filled with plenty of activities, participation and discussions. I am very proud to say that this current group of students that had just completed the said workshop is one of the best batch i have ever coached. They didn’t know much about Mole Calculations (some call it Mole Concepts) when we first started, but their enthusiasm and willingness to learn makes it a joy to coach them. They were so responsive and highly motivated to do well for this voted No. 1 Killer Topic in GCE ‘O’ Levels (and IP and IGCSE) Chemistry. Students were from the following secondary schools:
- Cedar Girls’ Secondary School
- Clementi Town Secondary School
- Dunman Secondary School
- Junyuan Secondary School
- Outram Park Secondary School
- Raffles Girl’s School
- St. Andrew’s Secondary School
- St. Hilda’s Secondary School
- St. Margaret’s Secondary School
- Tanjong Katong Girl’s School
- Whitley Secondary School
It is wonderful to know that we have coached students from more than 80 Secondary Schools in Singapore as well as a couple of them from overseas ie, malaysia, indonesia, mauritius, etc – making a difference to their lives through our coaching.
It is always great to know that the participants felt good excellent motivated & confident after the completion of each workshop. Here is a few thoughts on recent Mole Calculations Mastery Workshop:
“Personally I feel this workshop has helped me a lot as before I started if I was lost in the chapter of Mole Calculations. But now after attending this workshop, it has gave me a lot of confidence to face Mole questions. I would be happy to recommend this programme to my fellow friends as I think it would help them out too!”
Jedaiah Gwee, St. Andrew’s Secondary School
“Through this programme, I have literally had tremendous improvement for Mole Calculations =) In the past, I really couldn’t get a single questions on
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In Mole Calculations, many students lose marks in questions related to Volumetric Analysis, especially on those that requires them to understand the concepts of CONCENTRATIONS of solutions. Volumetric Analysis are always a highlight in our annual Mole Calculations Mastery Workshop.
Concentrations of a solution refers to the amount of solute dissolved in 1 dm3 of the solution.
Do note that:
Solution = Solute (minor component) + Solvent (major component)
Usually in basic chemistry such as O Levels, the solvent used is Water. As such, the solution formed is usually aqueous solutions whereby certain solid or liquid are dissolved in water.
Concentrations can be expressed in:
- Grams of solute per dm3 (unit will be g/dm3)
- Moles of solute per dm3 (unit will be mol/dm3)
The most important formulae student need to know in volumetric analysis are:
Conc. (mol/dm3) = Amt. of Solute (mol) / Vol. of Solution (dm3)
&
Conc. (g/dm3) = Amt. of Solute (g) / Vol. of Solution (dm3)
Let’s check out a question to see how we can use the formulae above.
Question:
60g of NaOH is dissolved in water and make up to the total volume of 500cm3. Calculate the
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